Horn doesn't work and not sure where to start. Switch or horn itself? If it is horn itself, where can I get replacement?

Take horn out (pop plastic cover and remove two screws) and see of you have power up in the bow while someone else hits the switch. Sometimes the connections get corrosion on them.

Horn itself is easily tested while out with any two wires directly to the battery.

I keep a piece of speaker wire on the boat for little annoying stuff like this. That way you can always bypass fuses, grounds, etc. For testing.

If you need to replace the horn it is very common. Just google boat horn. Measure distance between two screws that hold it in. That's how they are sold.